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National Conference On Electrical Sciences -2012 (NCES-12) ISBN: 978-93-81583-72-2

Department Of EEE, Annamacharya Institute Of Technology & Sciences, Rajampet 67

Control Scheme of Multi level Cascaded

H-Bridge STATCOM

Gollapalle Pullaiah S.Sarada O. HemakesavuluM.Tech Assistant Professor Associate Professor

AITS, Rajampet AITS, Rajampet AITS, Rajampet

Kadapa, India Kadapa, India Kadapa, India

[email protected] [email protected] [email protected]

AbstractThis paper presents a control scheme of

cascaded H-bridge STATCOM in three-phase power

systems. Cascaded H-bridge STATCOM has merits in

point of switching losses, output harmonics, and the

number of circuit components. But every H-bridge cell

has isolated dc capacitors. So the balancing problem of

capacitor voltages exists. Since STATCOM is often

requested to operate under asymmetrical condition bypower system faults, capacitor voltage balancing

between phase clusters is particularly important.

Solving this problem, a technique using zero-sequence

voltage and negative-sequence current is proposed.By

this scheme, the STATCOM is allowed to operate under

asymmetrical conditions by power system faults. The

validity is examined by digital simulation under one line

and two-lines fault circuit condition.

Index TermsCapacitor voltage balancing, cascaded H-

bridge, multilevel converter, negative-sequence current,

STATCOM, zerosequence voltage.

I. INTRODUCTIONCASCADED H-bridge multilevel converter consists ofseries connected H-bridge cells. It has merits of switching

losses of semiconductor device and harmonics in outputvoltage. And it is considered to be suitable for STATCOMin power system application, because it requires lessnumber of circuit components compared with diode-

clamped multilevel converter or flying capacitor multilevelconverter and STATCOM does not have to handle real

power [1][3]. But every H-bridge cell has isolated dc

capacitor and balancing problem of capacitor voltagesexists in this configuration [3][8]. STATCOM is often

requested to operate under asymmetrical condition bypower system faults, such as one line grounding or two-lines short circuit [9]. These kinds of faults cause unbalance

of power system voltage [10] and unbalance current flowsinto each phase cluster. So capacitor voltage balancingbetween phase clusters is particularly important. Recently,several methods of voltage balancing between phase

clusters are proposed [4], [5]. One method is based on zero-sequence voltage injection [4]. However it needs widemargin of dc capacitor voltage compared with rated powersystem voltage when the unbalance of power system

voltage is large. The other method handles the capacitorvoltage unbalance by independently controlling active

power of individual phase cluster, but unbalance of power

system voltage is not considered [5]. By these reasons, thecircuit condition in which these methods are effective isconsidered to be limited in practical use.We also had

proposed a capacitor voltage balancing method using

negative-sequence current [11]. It does not need widemargin of dc capacitor voltage and can handle large

unbalance of power system voltage. However the outputcurrent of the STATCOM using the method is uniquelydetermined by the unbalance of power system voltage and

function of the STATCOM is limited. So we introduce a

different control method using zero-sequence voltage inthis paper. By this method, the STATCOM can controloutput current almost freely. But it needs wide margin of dc

voltage under large power system voltage unbalance,similar to the method proposed in [5]. To avoid this, weexclusively use the two methods depending on the extent ofvoltage unbalance. The validity is examined by digital

simulation under one line and two-lines fault circuitcondition.

II. MAIN CIRCUIT AND CONTROL SCHEME

A. Main Circuit and Basic Operation

Fig. 1(a) shows the main circuit of cascaded H-bridge

STATCOM in this paper [11]. It is composed of

three-phase clusters. Each phase cluster consists of

three H-bridge cells. The dc capacitor voltages are set

to Vc,2vc , 4vc, in a phase cluster. Fig. 1(b) showsan example of output waveform. Voltage level from

- 7 to +7 can be generated by combining the

capacitor voltages. The level is decided according to

the calculation flow shown in Fig. 1(c). Then the

cluster outputs the nearest voltage level to reference.

Conventional cascaded H-bridge multilevel converter

may require high number of H-bridge cells for low

current distortion.But the proposed circuit

configuration can output 15-level voltage in spite of

only three cells. So, lower conduction losses of

semiconductor devices are expected.For dc voltage

balancing in each phase cluster, the controlmethod proposed here uses the fact that several switching

patterns are available when a phase cluster outputsparticular voltage levels. An example is shown in Fig. 2.

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When a phase cluster outputs voltage Vc, 2Vc-Vc,4Vc-2Vc-Vc and charged or discharged capacitors aredifferent. These patterns are selected

Figure1 Circuit configuratio and operation, (a) Main circuit,(b)

Example of output waveform, (c) Decision method of output level.

according to the relation between Vc1, Vc2 , and Vc3.When 4Vc19FandVc3 , output pattern Vc isselected. When current

Figure2 An example of voltage balancing between the H-bridge cells

When and , output pattern 2Vc-Vc is selected. To use

same switching pattern in 1/4 cycle, the capacitor voltagesare measured [rad] of ac side phase angle. Table I showsthe all operational patterns and decision method. It is usedwhen the polarities of STATCOM output voltage andcurrent are same. 1 indicates that the cell outputs voltage

to positive direction and its capacitor is discharged. indicates that the cell output voltage to negative directionand its capacitor is charged. It is similar when the polarityof the output current is opposite. By this method, capacitor

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voltage ratio between H-bridge cells in a phase cluster iscontrolled.

Table I shows the all operational patterns and decisionmethod. It is used when the polarities of STATCOM outputvoltage and current are same. 1 indicates that the cell

outputs voltage to positive direction and its capacitor isdischarged. -1 indicates that the cell output voltage tonegative direction and its capacitor is charged. It is similarwhen the polarity of the output current is opposite. By thismethod, capacitor voltage ratio between H-bridge cells in a

phase cluster is controlled.

B. Capacitor Voltage Balancing Between Phase Clusters:STATCOM is often requested to operate under

symmetrical condition by power system faults, such as oneline grounding or two-lines short circuit. These kinds offaults cause unbalance of power system voltage andunbalance current flows into each phase cluster. Then

capacitor voltage unbalance between phase clusters occurs.So we had proposed a capacitor voltage balancing methodusing negative-sequence current [11]. The negative-sequence current ,Ina, Inb, Inc , for capacitor voltage

balancing is expressed as

Here, it is assumed that the STATCOM shown in Fig. 1

operates under the asymmetrical circuit condition, as shown

And its output current is controlled as shown in (4). Where,the first term is the active current to compensate converterlosses. The second term is the reactive current output to

power system. The third term is the negative-sequence

current for voltage balancing, as shown in (1)

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Then, the average real powers of each phase clusters are

calculated as

In (5), the first term is proportional to the error between theindividual capacitor voltage ,Vca,Vcb,Vcc , and theaverage capacitor voltage Vc expressed as (6) and (2). The

second term is the same for each phase cluster. The thirdterm is almost independent of Vca, Vcb,Vcc,as expressedin (8). So the error of the individual capacitor voltage isdecreased by use of the negative-sequence currentshown in

So PI controller is actually used to calculate , in the controlblock of proposed STATCOM, instead of gain , to makethe error between the individual capacitor voltage and theaveragecapacitor voltage zero, as shown in Fig. 3.Howeverthe output current of the STATCOM using the negative-

sequence current method is uniquely determined byunbalance of power system voltage and function of theSTATCOM is limited. For example, the STATCOM

becomes impossible to compensate negative-sequencecurrent by unbalanced loads. So we introduce a different

control method using zero-sequence voltage in this paper.The zero-sequence voltage for capacitor voltage balancing

is expressed

where and are arbitrary phasor in (10) and andare their instantaneous value. is angular frequency of powersystem.By using shown in (10), STATCOM output power

from each phase is calculated as follows. See equation (12)at the bottom of the page. From (12), it is understood thatthe STATCOM outputs same power from each phase whenthe capacitor voltage of each phase is balanced. Even if

capacitor voltage unbalance occurs, it is corrected by thesecond term of (12). It can be easily confirmed that the timeconstant to correct the capacitor voltage unbalance is .However, this method requires a wide dc voltage margin.

For example, it is assumed that two-lines short circuitoccurs in

V0 is calculated as follows

Then statcom output voltages

As mentioned before, the STATCOM using must output

1.5 times voltage compared with only using positive andnegativesequence voltage at the circuit condition shown in(13). To avoid this, we exclusively use the two voltage

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balancing methods depending on the extent of voltageunbalance. The zero-sequence voltage method is usednormally. The negative- sequence current method is used

under large unbalance of power system voltage. Thedecision method shown in Fig. 4 is used to select these twomethods. Input is capacitor voltage necessary foroutputting reference voltage of the STATCOM. The

calculation method of is described in next section. Input , is

the negative-sequence component of power system voltageon reverse rotated frame, described in the next section.

Control scheme of cascaded H-bridge statcom

C. Control Scheme for STATCOMFig. 5 shows the control block of the STATCOM shown inFig. 1. The STATCOM is designed to control the positive

sequence voltage at the grid connection point to referenceby -axis current .The average capacitor voltage is ontrolledto reference by -axis current . Where, is from (6) and (2).The dc voltage is varied according to the STATCOM

output voltage shown in Fig. 6. The reason why this

method is adopted is to use as many voltage levels of thecascade H-bridge multilevel converter as possible,regardless of the peak of STATCOM output voltage. In

addition, the control element 7/6.5 shown in Fig. 6 actsto set the peak of reference in the middle of output level 6

and 7. The zero-sequence voltage shown in (10) or thenegative sequence current , shown in Fig. 3 are used for

capacitor voltage balancing between phase clusters. If overvoltage flag shown in Fig. 4 is set to 0, then is transformedto

The detection method of the grid voltage, shown in underpart of Fig. 5, is designed to control the STATCOM outputcurrent accurately under asymmetrical circuit conditions

by power system faults. The grid voltage are once

decomposed to positive sequence , and negative sequence ,by the method shown in Fig. 7. The control element lag90 delays the input , for 1/4 cycle at fundamentalfrequency of ac voltage and outputs For example,these

are expressed as

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The feature of this method is that the appropriate values areobtained in about 1/4 cycle even if sudden change of occurs

by power system faults. After transformation and low passfiltering, the positive sequence other hand are once rotated

to reverse direction of transformation. Here, the negative-sequence voltage is also obtained as dc components. After

low pass filtering, the output values are rotated two times toforward direction of transformationand the negative-sequence voltage are obtained accurately.The time constant of low pass filter does not have to be

long, ie. the delay time of low pass filter is not long,because the filter is requested to eliminate only harmoniccomponent of ac side voltage. As a result, the STATCOMcan respond to power system faults quickly, and the error

of capacitor voltage between phase clusters is expected tobe small even in the transient stateby the faults. In addition, these are used for PLL, shown inFig. 8, to synchronize the phase angle to positive sequence

of the grid voltage. And the positive sequence voltage atgrid point is also obtained by this control block diagram.

III. SIMULATION RESULT

Digital simulation using EMTP (Electro MagneticTransient Analysis Program) has been carried out to verifythe effectiveness of the proposed scheme for the circuitshown in Fig. 1. The TABLE III CONTROL

PARAMETERS

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circuit parameters are given in Table II. Rated reactivepower is 1 MVA. The sum of rated capacitor voltage is825V+1650V+3300V =5575V slightly higher than the peak

voltage of ac system which is .5389V The capacitance 1mF, 2 mF, 4 mF are chosen so that the capacitor voltageripple are less than about 5% of their rated voltage atoutputting rated reactive power. The ac reactance 11.6 mH

is equivalent to 10% at rated reactive power. The control

parameters are given in Table III.

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The simulation result of power system voltage andcapacitor voltages are shown in Fig. 10. The STATCOMcompensates power system voltage. As a result, grid

connection voltage is 263 V higher than source voltageduring 1LG and 2LS. Before 1LG the capacitor voltages of

phase clusters are balanced and the voltage ratio of H-bridge cells is controlled to 1:2:4. At the starting point of

1LG and 2LS, the capacitor voltages of phase clusters are

once unbalanced. But they are rebalanced soon. Fig. 11shows the power system voltage, STATCOM outputvoltage, STATCOM output current and capacitor voltages.

Under normal condition or 1LG the zero-sequence voltagemethod is used and balanced current are output, as shownin Fig. 11(a). The peak value of the currents is about 120 A.On the other hand, the negative-sequence current method is

used and the STATCOM output unbalanced current under2LS, as shown in Fig. 11(b). By this, the capacitor voltagesare balanced. During this time, the peak value ofSTATCOM output voltage is about 7000 V. If the zero-

sequence voltage method was used under 2LS, the

STATCOM outputs balanced current, as shown in Fig. 12.But the peak value of the STATCOM output voltage isabout 8000 V. Thus, the STATCOM had to output high

voltage and a wide margin of dc capacitor voltage wasneeded. As described before, the combination of twocapacitor voltage method realizes reasonable circuit designand flexible function of the STATCOM.

IV. CONCLUSIONThis paper presented a configuration and control scheme ofcascaded H-bridge STATCOM in three-phase powersystem. We proposed a control method using zero-sequence voltage and negative-sequence current. The two

methods are used exclusively depending on the extent ofvoltage unbalance. By this method, STATCOM can operate

flexibly under normal power system condition and does notneed wide margin of dc capacitor voltage under large

asymmetrical condition. The validity is examined by digital

simulation under one line and two-lines fault circuitcondition. The simulation results showed the effectivenessof proposed STATCOM. In addition, proposed control

scheme can be used for other type of applications, such asPV(photovoltaic) inverter systems. It expands applicablescope of cascaded H-bridge multilevel converter.

REFERENCES

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[3] O. Alizadeh and S. Farhangi, Voltage balancingtechnique with low switching frequency for cascademultilevel active front-end, presented at the 7th Int. Conf.Power Electronics, Daegu, Korea, Oct. 2007.

[4] R. E. Betz and T. J. Summers, Using a cascaded H-bridge STATCOM for rebalancing unbalanced voltages,

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